Solder the surface mount devices, then the remaining components.
Inspect soldered connections for shorts.
Using a multimeter, check that there’s no shorts between the black, red, and white power leads.
Connect the power lead, and jack a bipolar LFO to the input and jack a multimeter or oscilloscope to the output and inverted output. With the knobs in the middle position (pointing up) the output should almost mirror the input, and the inverted output should be inverted.
Check that the knobs work properly. The “SCALE” knob should increase/decrease the amplitude, and the “OFFSET” knob should shift the input voltage positive and negative.